1. Field of the Invention
This present invention relates generally to a method and device for facilitating a uniform loading condition for a plurality of support members supporting a steam dryer in a nuclear reactor.
2. Description of the Related Art
A reactor pressure vessel (RPV) of a nuclear reactor such as a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the reactor core and is supported by a shroud support structure. The shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically-shaped shroud.
Heat is generated within the core and water circulated up through the core is at least partially converted to steam. Steam separators separate the steam and the water. Residual water is removed from the steam by steam dryers located above the core. The de-watered steam exits the RPV through a steam outlet near the vessel top head.
Conventional BWR design steam dryers may exhibit vibratory motion during operation, which manifests itself as unusual wear or in some cases cracking on upper bearing surfaces of steam dryer support brackets which support to steam dryer within the RPV. The wear and cracking may suggest that the dryer is rocking due to uneven contact with its support brackets.
FIG. 1 illustrates an upper portion of a conventional RPV of a BWR to illustrate a steam dryer arrangement within the RPV. Referring to FIG. 1, there is illustrated an RPV 10 with top head 15 secured to the RPV 10 via main closure head bolts 17. Some of the components within the RPV 10 of the conventional BWR have been removed for clarity, although FIG. 1 illustrates the head spray nozzle 18 penetrating the top head 15, a conventional steam dryer 30 for removing residual water entrained in steam exiting the steam separators (not shown), a steam dryer hold down assembly 40, and the steam outlet nozzle 25, which carries the de-watered steam exiting the steam dryer 30 out of the RPV 10 to power turbine generators and associated downstream electrical distribution systems. Residual water or condensate removed from the steam-water mix 35 exiting the steam dryers is returned to the reactor via drain channels 37 in the steam dryer 30.
The steam dryer hold down assembly 40 may include a plurality of RPV steam dryer hold down brackets 42 connected to the dryer 30 via a corresponding steam dryer lifting rod eye 44 to steam dryer lifting rod 46. A typical steam dryer hold down assembly for conventional BWR steam dryers 30 may include four RPV steam dryer hold down brackets 42 attached to the inside of the top head 15 of the RPV 10. During reactor construction, the corresponding steam dryer lifting rod eyes 44 were typically adjusted so that their upper surfaces had from about one-half inch to one inch clearance to the bottom surface of the steam dryer hold down brackets 42, as shown generally by element number 48 in FIG. 1. This clearance gap 48 was provided to allow for differential thermal expansion between the RPV 10 and the steam dryer lifting rod 46. The dryer hold down assembly 40 was intended to function only during a steam line break accident (i.e., loss-of-coolant accident (LOCA)), where the dryer 30 lifts completely off of its support brackets 50, which support dryer support ring 60 attached to the steam dryer 30. Thus, the dryer hold down assembly 40 is not effective as a stabilizing support to prevent the vibratory motion of the steam dryer 30 during normal operation.
Also, there is concern that operating reactor plants implementing uprated power operations (power operations in excess of rated power output) may be more susceptible to this vibratory motion, due to increased steam flow and pressure drop across the dryer 30. As a result, dryer rocking tests are being recommended in conjunction with power uprates, even if a given, evaluated plant has not previously experienced this problem.
There may be an additional concern in a plant where the steam dryer is to be replaced due to damage in service. Here, an effective means of establishing uniform support for the steam dryer in the fit-up of a new steam dryer may be desired.
Conventionally, where uneven contact with the support brackets 50 is observed, a steam dryer rocking test is used to evaluate required corrective action. The amount and location of the non-uniformity at the interfaces which support the dryer 30 and the dryer support ring 60 between the respective mating steam dryer support brackets 50 which are attached to the RPV upper shell 20 is determined by test. The test is performed by inducing rocking of the dryer 30, first about a “north/south” or first axis, then an “east/west” or second axis. This test was performed by inserting a shim in one support bracket 50 location, and applying a tipping weight on the top of the dryer 30 at one eccentrically positioned location, then the diagonally opposite location. The amount of tipping is measured using dial indicators that record the relative movement of the tops of the dryer lifting rods.
This testing procedure is repeated with the shim and weight positions relocated (about 90 degrees) to the next axis. As the measured values include effects of sagging due to dryer 30 flexibility and non-uniform spacing of the support brackets 50, the needed adjustments must be approximated by complex indirect calculations. If required, adjustment is typically made by grinding the bottom of one or more dryer support ring 60 contact or bearing surfaces. As no further reactor maintenance can be performed within the RPV until the dryer is removed, reducing the critical path duration required for dryer support measurements is highly desirable.